Regarding a motor having permanent magnets pasted on a rotor surface (hereinafter called "surface permanent magnet type motor"), a conventional method of reducing torque ripples is described as follows:
When the motor is not yet powered on, either a shape of a permanent magnet which is to be pasted on a surface of a rotor or a shape of a stator core is modified so that a waveform of an induced voltage which occurs at stator windings can be a sine wave when a rotor is rotated by some means from outside. Then, a driving current of a sine wave is applied to the windings, thereby reducing a torque ripple.
FIG. 17 is a cross sectional view of the surface permanent magnet type motor which adopts the above method to reduce torque ripples. In FIG. 17, a shaft 24 pierces through and is fixed to the center of rotor core 21 comprising a laminated steel plate. A permanent magnet 22 is pasted with glue or the like on the surface of the rotor core 21. A shape of the permanent magnet is as follows: both of its inner and outer diameters are convex arc toward outside, and the radial center width is wider than those of both ends. The surface permanent magnet type rotor 20 comprises the above elements, i.e., the rotor core 21, magnet 22 and shaft 24.
A stator 11 has a plurality of teeth 12, and the teeth are provided with windings (not shown.) The rotor 20 faces to the stator 11 with an air gap having a narrow clearance in between.
When magnetic flux produced from the magnet 22 runs to the teeth 12 of the stator 11, quantity of magnetic flux varies moderately due to the above rotor's structure, whereby an induced-voltage-waveform produced in the windings can be approximated to a sine wave. Then, a current of sine wave is applied to the above winding, thus the torque ripple can be reduced.
However, when this type of motor is rotated at a high speed, the magnets 22 pasted to the rotor surface scatter due to centrifugal force, thus some measure is required such as covering the rotor 20 with a tube made of stainless steel.
Regarding a reluctance motor having no permanent magnet, a current pattern which can narrow spread of the produced torque is established in the current supplied to the windings, and then the torque is controlled based on the pattern of the supplied current. This method is laid open in the Japanese Patent Application non-examined publication No. H02-206389.
These kinds of motors are indeed strongly built; however, they sometimes produce insufficient torque due to no magnet.
In order to overcome these problems, a motor with interior permanent magnets has been recently commercialized. This motor incorporates the permanent magnets inside the rotor core, thereby realizing high efficiency and a high torque.
FIG. 5 is a cross sectional view depicting a structure of the motor having interior permanent magnets, and FIG. 6 illustrates the torque produced by this kind of motor, where the X-axis indicates a current phase supplied to the stator windings and the Y-axis indicates a magnitude of the torque. In FIG. 6, a curve 51 represents a torque produced by the magnet (hereinafter called "magnet torque"), a curve 52 represents a reluctance torque and a curve 53 represents a combined torque of these two.
In the structure shown in FIG. 5, the following relationship is established : EQU Ld&lt;Lq
where Ld is an inductance along "d" direction, and Lq is an inductance piercing the boundary of the rotor poles.
In general, the torque T of the motor is indicated by the following equation: EQU T=Pn{.psi.a.multidot.I.multidot.cos .beta.+0.5(Lq-Ld)I.sup.2 .multidot.sin 2.beta.}
where "Pn" is a number of pairs of the rotor poles, ".psi.a" is an interlinkage magnetic flux between the rotor and stator, "I" is a winding current of the stator, and ".beta." is a lead phase angle (electrical angle).
In the above equation, the first term represents a magnet torque and the second term represents a reluctance torque. Since Ld&lt;Lq is established in the above equation, the phase of winding current "I" is controlled to advance with regard to the phase of the induced voltage produced in each phase, whereby .beta.&gt;0 is realized and a reluctance torque is produced. Thus, comparing with the case with a magnet torque only, the above structure can produce a larger torque at the same current level by setting ".beta." at a predetermined value. When the motor having the above interior permanent magnets is driven by the sine wave current shown in FIG. 18, a torque ripple shown in FIG. 19 is produced. This torque ripple is combined by the magnet torque and reluctance torque.
As such, the motor having the interior permanent magnets has a difficulty in reducing the torque ripple because the produced torque is combined by the magnet torque and reluctance torque although the stator-winding-current is shaped into a sine wave.
The above description can be summarized as follows: The conventional surface-permanent-magnet type motor produces torque due to only the permanent magnets, therefore, the shape of the magnet is modified so that the waveform of induced voltage produced in the stator windings is approximated to a sine wave and the current of sine wave is supplied to the windings, thereby reducing the torque ripple. On the other hand, in the motor having the interior permanent magnets, the waveform of the induced voltage produced in the stator windings would be approximated to a sine wave, and the current of the sine wave would be supplied to the windings, the produced torque is combined by the magnet torque and reluctance torque. Therefore, even the torque ripple component due to the permanent magnet can be reduced, the other component due to the reluctance torque cannot be reduced.